The following aspects of our general efforts on the characterization of the components and pathways of mammalian electron transfer systems will be emphasized in 1973-74: 1.) Characterization, identification, and studies on the possible function(s) of a new type of electron carrier not previously found in mammalian tissues. 2.) Kinetic studies by rapid freeze-quenching and low temperature EPR spectroscopy on this electron carrier, on the iron-sulfur proteins of mammalian mitochondria, and on the electron carriers of cytochrome c oxidase in the reaction with oxygen. 3.) Correlation of growth state, oxidative phosphorylation at site I, and genesis of iron-sulfur proteins in Torula yeast. 4.) The dependence of various species of cytochrome P450 of rat adrenal mitochondria on hormonal control.